Electroactive polymers are now frequently used in a number of optical devices such as in polymeric light emitting diodes (“PLEDs”) as disclosed in WO 90/13148, photovoltaic devices as disclosed in WO 96/16449 and photodetectors as disclosed in U.S. Pat. No. 5,523,555.
A typical PLED comprises a substrate, on which is supported an anode, a cathode and an organic electroluminescent layer between the anode and cathode comprising at least one polymeric electroluminescent material. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light. Other layers may be present in the PLED, for example a layer of organic hole injection material such as poly(ethylene dioxy thiophene)/polystyrene sulfonate (PEDT/PSS) may be provided between the anode and the organic electroluminescent layer to assist injection of holes from the anode to the organic electroluminescent layer.
In a typical PLED, the electroluminescent material is provided as a single layer as described in, for example, WO 99/48160 which comprises a blend of a hole transporting polymer, an electron transporting polymer and an emissive polymer. Alternatively, a single polymer may provide two or more of the functions of hole transport, electron transport and emission. The electroluminescent polymer or polymers are preferably soluble in common organic solvents to facilitate their deposition. One such class of soluble polymers are polyfluorenes which have good film forming properties and which may be readily formed by Suzuki or Yamamoto polymerisation which enables a high degree of control over the regioregularity of the resultant polymer.
It may, however, be preferred to cast multiple layers, i.e., laminates, of different polymers on a single substrate surface, so that one can achieve optimisation of separate functions, for example electron or hole charge transport, luminescence, photo-induced charge generation, and charge blocking or storage. Furthermore, it is believed that PEDT/PSS may have a deleterious effect on the electroluminescent layer, for example by ingress of protons or sulfonate groups from PSS into the electroluminescent layer (i.e. the layer in which holes and electrons combine to form an exciton), resulting in quenching of luminescence. Accordingly, it may be desirable to provide a protective layer between PEDT/PSS and the electroluminescent layer. However, preparation of polymer laminates can be problematic due to solubility of initially cast or deposited layers in the solvents used for succeeding layers.
Layers of electroluminescent polymer may be formed by depositing a soluble polymeric precursor which is then chemically converted to an insoluble, electroluminescent form. For example, WO 94/03030 discloses a method wherein insoluble, electroluminescent poly(phenylene vinylene) is formed from a soluble precursor and further layers are then deposited from solution onto this insoluble layer. However, the chemical conversion process involves extreme processing conditions and reactive by-products that may harm the performance of the finished device. Accordingly, electroluminescent polymers that are soluble in common organic solvents are preferable. Examples of such materials are disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and include polymers with fully or at least partially conjugated backbones such as polyfluorenes, polyphenylenes and poly(arylene vinylenes) with solubilising groups, and polymers with non-conjugated backbones such as poly(vinyl carbazole).
WO 98/05187 discloses a method of forming a multilayer device comprising the steps of depositing a poly(vinyl pyridine) layer onto PEDT/PSS and depositing a PPV precursor onto the poly(vinyl pyridine) layer. As above, this precursor requires harsh processing conditions in order to convert to a semiconducting material.
U.S. Pat. No. 6,107,452 discloses a method of forming a multilayer device wherein fluorene containing oligomers comprising terminal vinyl groups are deposited from solution and cross-linked to form insoluble polymers onto which additional layers may be deposited. Similarly, Kim et al, Synthetic Metals 122 (2001), 363-368 discloses polymers comprising triarylamine groups and ethynyl groups which may be cross-linked following deposition of the polymer. The choice of polymers in both instances is constrained by the requirement that a plurality of vinyl or ethynyl moieties be present.
IEEE Transactions on Electron Devices, 44(8), 1263-1268, 1997 discloses the formation of a bilayer of poly(vinyl carbazole) PVK and a pyridine-containing conjugated polymer. This is possible because the solvent used to deposit the pyridine-containing conjugated polymer does not dissolve the underlying layer of PVK.
J. Liu, Z. F. Guo and Y. Yang, J. Appl Phys. 91, 1595-1600, 2002, discloses forming a multilayer device by depositing and heating a semiconducting polymer and then depositing another layer of the same polymer.
WO 99/48160 discloses PLEDs wherein a layer of hole transport material is provided between a layer of PEDT/PSS and a layer of electroluminescent material.
Although much effort has been devoted to the fabrication of multilayered polymeric optical devices, including PLEDs, there still exists a continued need to provide multilayered polymeric optical devices having improved performance, and methods to produce such devices.